This invention relates to an air/fuel ratio control apparatus for internal combustion engines.
Conventionally, an air/fuel ratio control apparatus uses a conventional oxygen sensor, the output signal of which changes stepwise between upper and lower levels at an air/fuel ratio .lambda. equal to one (stoichiometric air/fuel ratio), as described in JP-A-51-106828. Accordingly, if the conventional air/fuel ratio control apparatus is employed with another type of air/fuel ratio sensor whose output changes in proportion to the air/fuel ratio, the control system will operate erroneously. More particularly, in the sensor producing an output signal proportional to the air/fuel ratio, the gain relative to the air/fuel ratio is smaller in the lean region and larger in the rich region. Therefore, if the proportional constant of the control system, for example, remains unchanged throughout the rich and lean regions, an erroneous operation, such as hunting, will result within either one of the rich and lean regions.